Factors affecting the internal resistance of lithium-ion batteries

Factors affecting the internal resistance of lithium-ion batteries

With the use of lithium batteries, their performance continues to decline, mainly manifested as capacity decay, internal resistance increase, power decrease, etc. The changes in battery internal resistance are influenced by various usage conditions such as temperature and discharge depth. Therefore, the factors that affect the internal resistance of the battery were elaborated in terms of battery structure design, raw material performance, manufacturing process, and usage conditions.

Resistance is the resistance experienced by the current flowing through the interior of a lithium battery during operation. Usually, the internal resistance of lithium batteries is divided into ohmic internal resistance and polarized internal resistance. Ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance, and contact resistance of various parts. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reactions, including electrochemical polarization internal resistance and concentration polarization internal resistance. The ohmic internal resistance of a battery is determined by the total conductivity of the battery, and the polarization internal resistance of the battery is determined by the solid-state diffusion coefficient of lithium ions in the electrode active material.

Ohmic Resistance

Ohmic internal resistance is mainly divided into three parts: ion impedance, electron impedance, and contact impedance. We hope that the internal resistance of lithium batteries will decrease as they become smaller, so specific measures need to be taken to reduce the Ohmic internal resistance based on these three aspects.

Ion impedance

The ion impedance of a lithium battery refers to the resistance experienced by the transmission of lithium ions within the battery. The migration speed of lithium ions and electron conduction speed play equally important roles in lithium batteries, and the ion impedance is mainly influenced by the positive and negative electrode materials, separators, and electrolyte. To reduce ion impedance, the following points need to be done well:

Ensure that the positive and negative electrode materials and electrolyte have good wettability

When designing the electrode, it is necessary to select an appropriate compaction density. If the compaction density is too high, the electrolyte is not easy to soak and will increase the ion impedance. For the negative electrode, if the SEI film formed on the surface of the active material during the first charge and discharge is too thick, it will also increase the ion impedance. In this case, it is necessary to adjust the battery formation process to solve the problem.

The influence of electrolyte

The electrolyte should have appropriate concentration, viscosity, and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between it and the active substances of the positive and negative electrodes. At the same time, the electrolyte also requires a lower concentration, which is also unfavorable for its flow and infiltration if the concentration is too high. The conductivity of the electrolyte is the most important factor affecting ion impedance, which determines the migration of ions.

The Effect of Diaphragm on Ion Impedance

The main influencing factors of the membrane on ion impedance include: electrolyte distribution in the membrane, membrane area, thickness, pore size, porosity, and tortuosity coefficient. For ceramic diaphragms, it is also necessary to prevent ceramic particles from blocking the pores of the diaphragm, which is not conducive to the passage of ions. While ensuring that the electrolyte fully infiltrates the membrane, there should be no residual electrolyte left in it, reducing the efficiency of electrolyte use.

Electronic impedance

There are many factors that affect electronic impedance, and improvements can be made from aspects such as materials and processes.

Positive and negative electrode plates

The main factors that affect the electronic impedance of positive and negative electrode plates are: the contact between the live material and the collector, the factors of the live material itself, and the parameters of the electrode plate. The living material needs to have full contact with the collector surface, which can be considered from the adhesion of the collector copper foil, aluminum foil substrate, and the positive and negative electrode slurry. The porosity of the living material itself, surface by-products of particles, and uneven mixing with conductive agents can all cause changes in electronic impedance. The parameters of the electrode plate, such as low density of live matter and large particle gaps, are not conducive to electron conduction.

Separators

The main influencing factors of the diaphragm on electronic impedance include: diaphragm thickness, porosity, and by-products during the charging and discharging process. The first two are easy to understand. After disassembling the battery cell, it is often found that there is a thick layer of brown material on the diaphragm, including graphite negative electrode and its reaction byproducts, which can cause blockage of the diaphragm hole and reduce the battery life.

Fluid collecting substrate

The material, thickness, width, and degree of contact between the collector and the electrode can all affect the electronic impedance. Fluid collection requires the selection of substrate that has not been oxidized or passivated, otherwise it will affect the impedance size. Poor soldering between copper aluminum foil and electrode ears can also affect electronic impedance.

Contact impedance

The contact resistance is formed between the contact of copper aluminum foil and live material, and it is necessary to focus on the adhesion of the positive and negative electrode paste.

Polarization internal resistance

The phenomenon of electrode potential deviating from the equilibrium electrode potential when current passes through the electrode is called electrode polarization. Polarization includes ohmic polarization, electrochemical polarization, and concentration polarization. Polarization resistance refers to the internal resistance caused by polarization between the positive and negative electrodes of a battery during electrochemical reactions. It can reflect the consistency within the battery, but is not suitable for production due to the influence of operations and methods. The polarization internal resistance is not a constant and constantly changes over time during the charging and discharging process. This is because the composition of active substances, the concentration and temperature of the electrolyte are constantly changing. Ohmic internal resistance follows Ohmic law, and polarization internal resistance increases with increasing current density, but it is not a linear relationship. It often increases linearly with the logarithm of the current density.

Structural design impact

In the design of battery structures, in addition to riveting and welding of the battery structural components themselves, the number, size, position, and other factors of the battery ear directly affect the internal resistance of the battery. To a certain extent, increasing the number of pole ears can effectively reduce the internal resistance of the battery. The position of the pole ear also affects the internal resistance of the battery. The winding battery with the pole ear position at the head of the positive and negative pole pieces has the highest internal resistance, and compared to the winding battery, the stacked battery is equivalent to dozens of small batteries in parallel, and its internal resistance is smaller.

Raw material performance impact

Positive and negative active materials

The positive electrode material in lithium batteries is the one that stores lithium, which determines the performance of the battery more. The positive electrode material mainly improves the electronic conductivity between particles through coating and doping. The doping of Ni enhances the strength of P-O bonds, stabilizes the structure of LiFePO4/C, optimizes the cell volume, and effectively reduces the charge transfer impedance of the positive electrode material. The significant increase in activation polarization, especially in negative electrode activation polarization, is the main reason for severe polarization. Reducing the particle size of the negative electrode can effectively reduce the activation polarization of the negative electrode. When the solid particle size of the negative electrode is reduced by half, the activation polarization can be reduced by 45%. Therefore, in terms of battery design, research on the improvement of positive and negative electrode materials themselves is also essential.

Conductive agent

Graphite and carbon black are widely used in the field of lithium batteries due to their excellent performance. Compared to graphite type conductive agents, adding carbon black type conductive agents to the positive electrode has better rate performance of the battery, because graphite type conductive agents have a flake like particle morphology, which causes a significant increase in pore tortuosity coefficient at high rates, and is prone to the phenomenon of Li liquid phase diffusion limiting discharge capacity. The battery with CNTs added has a smaller internal resistance because compared to the point contact between graphite/carbon black and the active material, the fibrous carbon nanotubes are in line contact with the active material, which can reduce the interface impedance of the battery.

Collecting fluid

Reducing the interface resistance between the collector and the active material and improving the bonding strength between the two are important means to improve the performance of lithium batteries. Coating conductive carbon coating on the surface of aluminum foil and conducting corona treatment on the aluminum foil can effectively reduce the interface impedance of the battery. Compared to conventional aluminum foil, using carbon coated aluminum foil can reduce the internal resistance of the battery by about 65% and reduce the increase in internal resistance during use. The AC internal resistance of aluminum foil treated with corona can be reduced by about 20%. In the commonly used range of 20% to 90% SOC, the overall DC internal resistance is relatively small and its increase gradually decreases with the increase of discharge depth.

Separators

The ion conduction inside the battery depends on the diffusion of Li ions through the porous membrane in the electrolyte. The liquid absorption and wetting ability of the membrane is the key to forming a good ion flow channel. When the membrane has a higher liquid absorption rate and porous structure, it can improve conductivity, reduce battery impedance, and improve the rate performance of the battery. Compared to ordinary base membranes, ceramic membranes and coated membranes can not only significantly improve the high-temperature shrinkage resistance of the membrane, but also enhance its liquid absorption and wetting ability. Adding SiO2 ceramic coatings on PP membranes can increase the liquid absorption capacity of the membrane by 17%. Apply 1 on the PP/PE composite membrane μ The PVDF-HFP of m increases the suction rate of the membrane from 70% to 82%, and the internal resistance of the cell decreases by more than 20%.

The factors that affect the internal resistance of batteries in terms of manufacturing process and usage conditions mainly include:

Process factors influence

Slurries

The uniformity of slurry dispersion during slurry mixing affects whether the conductive agent can be uniformly dispersed in the active material and closely contacts it, which is related to the internal resistance of the battery. By increasing high-speed dispersion, the uniformity of slurry dispersion can be improved, resulting in a smaller internal resistance of the battery. By adding surfactants, the uniformity of the distribution of conductive agents in the electrode can be improved, and electrochemical polarization can be reduced to increase the median discharge voltage.

Coating

Surface density is one of the key parameters in battery design. When the battery capacity is constant, increasing the electrode surface density will inevitably reduce the total length of the collector and separator, and the Ohmic internal resistance of the battery will also decrease. Therefore, within a certain range, the internal resistance of the battery decreases with the increase of surface density. The migration and detachment of solvent molecules during coating and drying are closely related to the temperature of the oven, which directly affects the distribution of adhesives and conductive agents within the electrode, thereby affecting the formation of conductive grids within the electrode. Therefore, the temperature of coating and drying is also an important process for optimizing battery performance.

Roller pressing

To a certain extent, the internal resistance of the battery decreases with the increase of compaction density, as the compaction density increases, the distance between raw material particles decreases, the more contact between particles, the more conductive bridges and channels, and the battery impedance decreases. The control of compaction density is mainly achieved through rolling thickness. Different rolling thicknesses have a significant impact on the internal resistance of batteries. When the rolling thickness is large, the contact resistance between the active substance and the collector increases due to the inability of the active substance to roll tightly, resulting in an increase in the internal resistance of the battery. And after the battery cycle, cracks appear on the surface of the positive electrode of the battery with a larger rolling thickness, which will further increase the contact resistance between the surface active substance of the electrode and the collector.

Pole piece turnover time

The different shelving times of the positive electrode have a significant impact on the internal resistance of the battery. The shelving time is relatively short, and the internal resistance of the battery increases slowly due to the interaction between the carbon coating layer on the surface of lithium iron phosphate and lithium iron phosphate; When left unused for a long time (greater than 23 hours), the internal resistance of the battery increases more significantly due to the combined effect of the reaction between lithium iron phosphate and water and the bonding effect of the adhesive. Therefore, in actual production, it is necessary to strictly control the turnover time of electrode plates.

Injection

The ionic conductivity of the electrolyte determines the internal resistance and rate characteristics of the battery. The conductivity of the electrolyte is inversely proportional to the viscosity range of the solvent, and is also influenced by the concentration of lithium salts and the size of anions. In addition to optimizing the conductivity research, the amount of liquid injected and the soaking time after injection also directly affect the internal resistance of the battery. A small amount of liquid injected or insufficient soaking time can cause the internal resistance of the battery to be too high, thereby affecting the capacity of the battery.

Impact of usage conditions

Temperature

The influence of temperature on the size of internal resistance is obvious. The lower the temperature, the slower the ion transport inside the battery, and the greater the internal resistance of the battery. The impedance of batteries can be divided into bulk impedance, SEI film impedance, and charge transfer impedance. The bulk impedance and SEI film impedance are mainly influenced by the electrolyte ion conductivity, and their variation trend at low temperatures is consistent with the electrolyte conductivity variation trend. Compared to the increase in bulk impedance and SEI film resistance at low temperatures, the charge reaction impedance increases more significantly with decreasing temperature. Below -20 ℃, the charge reaction impedance accounts for almost 100% of the total internal resistance of the battery.

SOC

When the battery is at different SOC, its internal resistance size also varies, especially the DC internal resistance directly affects the power performance of the battery, which reflects the actual performance of the battery. The DC internal resistance of lithium batteries increases with the increase of the battery discharge depth DOD, and the internal resistance size remains basically unchanged in the 10% to 80% discharge range. Generally, the internal resistance increases significantly at deeper discharge depths.

Storage

As the storage time of lithium-ion batteries increases, the batteries continue to age and their internal resistance continues to increase. The degree of variation in internal resistance varies among different types of lithium batteries. After 9 to 10 months of storage, the internal resistance increase rate of LFP batteries is higher than that of NCA and NCM batteries. The increase rate of internal resistance is related to storage time, storage temperature, and storage SOC

Cycle

Whether it is storage or cycling, the impact of temperature on the internal resistance of the battery is consistent. The higher the cycling temperature, the greater the rate of increase in internal resistance. The impact of different cycle intervals on the internal resistance of batteries is also different. The internal resistance of batteries increases rapidly with the increase of charging and discharging depth, and the increase in internal resistance is directly proportional to the strengthening of charging and discharging depth. In addition to the influence of the depth of charge and discharge during the cycle, the charging cutoff voltage also has an impact: too low or too high the upper limit of the charging voltage will increase the interface impedance of the electrode, and too low the upper limit voltage cannot form a passivation film well, while too high the upper limit voltage will cause the electrolyte to oxidize and decompose on the surface of LiFePO4 electrode to form products with low conductivity.

Other

Automotive lithium batteries inevitably experience poor road conditions in practical applications, but research has found that the vibration environment has almost no effect on the internal resistance of lithium batteries during the application process.

Expectation

Internal resistance is an important parameter for measuring the power performance of lithium-ion batteries and evaluating their lifespan. The larger the internal resistance, the worse the rate performance of the battery, and the faster it increases during storage and cycling. The internal resistance is related to the battery structure, material characteristics, and manufacturing process, and varies with changes in environmental temperature and state of charge. Therefore, developing low internal resistance batteries is the key to improving battery power performance, and mastering the changes in battery internal resistance is of great practical significance for predicting battery life.